Geophysical surveys are used in exploration projects that require geological information. Such surveys provide data that can provide information about the structure and distribution of rock types and their contents. This information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Most oil companies rely on geophysical surveys to select sites in which to drill exploratory oil wells.
One form of geophysical surveying, electromagnetic (EM) surveying, generates low frequency EM signals that interact with the strata of interest. Such signals may be transmitted from an emitter cable towed behind a ship or other surface or subsurface marine vessel travelling across a body of water such as a lake or ocean. The generated EM signals propagate through the water and the strata underneath the body of water. As the EM signals interact with the water and strata, the combined resistivity of the water and strata (and to a lesser extent the air above the water) causes the EM signals to decay as they propagate away from the emitter. The resulting electric field present in the water is governed by a combination of the resistive properties of the water, the sub-bottom strata and the air. Differences in the resistivity of the strata along the survey line will translate into minute differences in this electric field. Sensors within a geophysical survey cable (which may also be towed by the ship) detect and measure these minute differences of the electrical fields in the water produced by the EM signals, and these EM field measurements may be used to identify resistivity contrasts that may be indicative of bodies of interest within the strata (e.g., oil and gas reservoirs).
Because of the small magnitude of the measured EM fields, other sources of EM signals can interfere with their detection and measurement. Such sources of interference include the Earth's magnetic field, as well as electromagnetic fields produced by electrical currents within underwater structures such as pipelines and subsea cables. The movement of the towed geophysical survey cable through such magnetic fields can induce electrical noise into the cable's conductors, as can time-variant electromagnetic fields originating from the aforementioned underwater structures. This superimposed electrical noise can be of a magnitude comparable to that of the detected electrical current, making it difficult to discriminate from the noise. Some existing techniques calculate and subtract out some of this noise based upon measurement of the survey cable's movement within the Earth's magnetic fields (see, e.g., U.S. Pat. No. 7,671,598 by Ronaess, et al., hereby incorporated herein by reference). Such solutions, however, require a large number of additional sensors along the length of the cable (e.g., magnetometers and accelerometers). Moreover, these solutions also do not account for artificial underwater electromagnetic sources.
It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims.